Due to the selective introduction of fluorine-substituted organic radicals, the compounds according to the invention have particularly favorable charge control properties. In electrophotographic recording processes, a "latent charge image" is produced on a photoconductor. This can be done, for example, by charging a photoconductor by means of a corona discharge and then subjecting the electrostatically charged surface of the photoconductor to imagewise exposure, in which, as a result of the exposure, discharge to the earthed support takes place at the exposed area. The "latent charge image" thus produced is then developed by applying a toner. In a subsequent step, the toner is transferred from the photoconductor, for example, to paper, textiles, films or plastic and fixed there by pressure, radiation, heat or solvent action. The photoconductor used is then cleaned and available for the next recording process.
A large number of patent publications describe the optimization of toners, in the course of which, inter alia, the effect of the toner binder (variation of resin/resin components or wax/wax components), the effect of control agents or other additives or the effect of carriers (in the case of two-component developers) and magnetic pigments (in the case of one-component developers) were investigated.
A measure of the toner quality is its specific charge q/m (charge per mass unit). In addition to the sign and level of the electrostatic charge, an important criterion of quality is that the desired charge level is reached rapidly and maintained over a fairly long period of activation. This is of central importance in practice, since the toner in the developing mixture may be exposed to a considerable activation time before being transferred to the photoconductor, due to the fact that in some cases it remains in the developing mixture for a period necessary for producing up to several thousand copies. Moreover, a further important criterion of suitability is the insensitivity of the toner to climatic influences, such as temperature and relative humidity.
Depending on the type of the process and the apparatus, either positively or negatively chargeable toners are used in copiers and laser printers.
To obtain electrophotographic toners or developers having either a positive or negative triboelectric charge, so-called charge control agents are often added. In addition to the sign of the charge control, the extent of the control effect is important, since higher efficiency makes it possible to use a small amount.
Since toner binders alone usually show a considerable change in the charge depending on the activation time, the function of a charge control agent is on the one hand to establish the sign and level of toner charge and to counteract the charge drift of the toner binder and on the other to make sure that the toner charge remains constant. Charge control agents which are unable to prevent a high charge drift (ageing) of the toner or developer after an extended period of use, which can even have the effect that the toner or developer experiences a reversal of charge, are therefore not suitable for practical application. Full-color copiers and laser printers operate by the trichromic principle, which requires an exact color match of the three basic colors (yellow, cyan and magenta) Even if only one of the three basic colors shows a slight shift in hue, it is absolutely necessary for the two other colors to shift their hue, in order to be able to produce full-color copies and prints which are identical to the original.
Due to this precise match of the coloristics of the individual colorants required in color toners, it is of essential importance that charge control agents themselves do not have any color whatsoever.
In color toners, the three toners yellow, cyan and magenta must be matched exactly not only with respect to the exactly defined coloristic requirements but also in regard to their triboelectric properties. This triboelectric matching is necessary, because in full-color printing or in full-color copying the three color toners (or four color toners, if black is included) have to be transferred successively in the same apparatus.
Colorants are known to have a lasting effect in some cases on the triboelectric charge of toners (H.-T. Macholdt, A. Sieber, Dyes & Pigments 9 (1988), 119-27, U.S. Pat. No. 4,057,426, EP-OS 247,576). Due to the different triboelectric effects of colorants and the resulting influence on the toner chargeability, which in some cases may be very considerable, it is not possible to add them to an already prepared toner base recipe as exclusive colorant. Rather, it may become necessary to prepare an individual recipe for each colorant, for which, for example, the type and amount of the required charge control agent must be specifically tailored. Accordingly, this procedure is complicated and adds to the difficulties already described in the case of color toners for process inks (trichromatic inks).
This therefore requires highly efficient colorless charge control agents which are capable of compensating the different triboelectric behavior of various colorants and providing the toner with the desired charge. In this manner, it is possible to use triboelectrically very different colorants on the basis of an already prepared toner base recipe in combination with one and the same charge control agent in the various required toners (yellow, cyan, magenta and, where appropriate, black).
The colorless charge control agents which have been claimed for electrophotographic toners and developers are ammonium (e.g. EP-OS 203,532, EP-OS 242,420, U.S. Pat. No. 4,683,188, U.S. Pat. No. 4,684,596, JP-OS 63/226,665) and phosphonium compounds (e.g. U.S. Pat. No. 3,893,935, U.S. Pat. No. 4,496,643, JP-OS 63/231,358), metal complexes and organometallic compounds (e.g. U.S. Pat. No. 4,656,112, German Offenlegungsschrift 3,144,017, JP-OS 61/236,557, JP-OS 62/287,262) and a number of other compounds (e.g. EP-OS 216,295).
Positive colorless charge control agents essentially comprise quaternary ammonium and phosphonium compounds and some organotin and organoantimony compounds. However, the previously known colorless charge control agents have a number of disadvantages, which severely restrict or in some cases even prevent their use in practice. Thus, the quaternary ammonium compounds, which are suitable usually, are difficult to disperse, can lead to an uneven charge of the toner for reasons not yet determined, and the toner charge produced by them is not stable over a longer period of activation, in particular at elevated temperature and relative humidity (EP-OS 242,420). Furthermore, these types of compounds are sensitive to light or mechanical effects (EP-OS 203,532, U.S. Pat. No. 4,683,188) and are thermally unstable and give decomposition products which can have a disadvantageous effect on the triboelectric charge of the toners (U.S. Pat. No. 4,684,596). Some of these compounds are not really colorless and have therefore only limited use in color toners. Moreover, they often show a wax-like behavior, are in some cases water-soluble and have low efficiency as charge control agents. Phosphonium salts of the conventional type have, in addition to the disadvantages already mentioned in the case of the ammonium compounds, lower efficiency as charge control agents and poorer charge control properties (U.S. Pat. No. 4,496,643, EP-OS 161,128) than ammonium compounds (see Comparative Example 1).
Organotin borate, such as described in European Patent 216,295, and organotin (JP-OS 62/287,262) and organoantimony compounds (JP-OS 61/236,557) are problematical due to their heavy metal content.
The negative colorless charge control agents which have been disclosed include a few ammonium (JP-OS 63/226,665) and phosphonium compounds (JP-OS 63/231,358) and a number of heavy metal compounds, specifically chromium, iron, cobalt (German Offenlegungsschrift 3,144,017) and zinc complexes (U.S. Pat. No. 4,656,112, JP-OS 62/127,754). The disadvantages and restrictions already mentioned above also apply to the ammonium and phosphonium compounds.
In addition to incorporation in the toner resin, charge control agents are also used for the coating of carriers. The same classes of compounds as for the incorporation in toner resins are employed, e.g. ammonium (JP-OS 61/258,270) and phosphonium compounds (JP-OS 61/260,258) and heavy metal complexes and organometallic compounds (JP-OS 61/147,261, JP-OS 61/259,267).
It can in general be stated that colorless charge control agents are known in principle not only for positive but also for negative toners, but are suitable for practical application only to a limited extent or not at all or are still largely in need of improvement. This is also demonstrated by the fact that in particular the ammonium and phosphonium compounds have hitherto not been accepted in practice.
Accordingly, the aim of the present invention was to find improved charge control agents which are distinguished by having themselves no color whatsoever, being more effective, being suitable in particular for the use in color toners and rapidly adjusting the electrostatic charge of toners and developers as a function of concentration. Furthermore, they should maintain the electrostatic charge at a constant value over an extended period of activation (no ageing), should operate problem-free at high relative humidity and be compatible and readily dispersible with conventional toner resins.